/*
 * Copyright (c) 2012-2017 Chad Austin
 *
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use, copy,
 * modify, merge, publish, distribute, sublicense, and/or sell copies
 * of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#pragma once

#include <algorithm>
#include <assert.h>
#include <cstdio>
#include <limits.h>
#include <limits>
#include <math.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>

#ifndef SAJSON_NO_STD_STRING
#include <string> // for convenient access to error messages and string values.
#endif

#if defined(__GNUC__) || defined(__clang__)
#define SAJSON_LIKELY(x) __builtin_expect(!!(x), 1)
#define SAJSON_UNLIKELY(x) __builtin_expect(!!(x), 0)
#define SAJSON_ALWAYS_INLINE __attribute__((always_inline))
#define SAJSON_UNREACHABLE() __builtin_unreachable()
#define SAJSON_snprintf snprintf
#elif defined(_MSC_VER)
#define SAJSON_LIKELY(x) x
#define SAJSON_UNLIKELY(x) x
#define SAJSON_ALWAYS_INLINE __forceinline
#define SAJSON_UNREACHABLE() __assume(0)
#if (_MSC_VER <= 1800)
#define SAJSON_snprintf _snprintf
#else
#define SAJSON_snprintf snprintf
#endif
#else
#define SAJSON_LIKELY(x) x
#define SAJSON_UNLIKELY(x) x
#define SAJSON_ALWAYS_INLINE inline
#define SAJSON_UNREACHABLE() assert(!"unreachable")
#define SAJSON_snprintf snprintf
#endif

/**
 * sajson Public API
 */
namespace sajson {

/**
 * Indicates a JSON value's type.
 *
 * In early versions of sajson, this was the tag value directly from the parsed
 * AST storage, but, to preserve API compabitility, it is now synthesized.
 */
enum type : uint8_t {
    TYPE_INTEGER,
    TYPE_DOUBLE,
    TYPE_NULL,
    TYPE_FALSE,
    TYPE_TRUE,
    TYPE_STRING,
    TYPE_ARRAY,
    TYPE_OBJECT,
};

namespace internal {

/**
 * get_value_of_key for objects is O(lg N), but most objects have
 * small, bounded key sets, and the sort adds parsing overhead when a
 * linear scan would be fast anyway and the code consuming objects may
 * never lookup values by name! Therefore, only binary search for
 * large numbers of keys.
 */
constexpr inline bool should_binary_search(size_t length) {
#ifdef SAJSON_UNSORTED_OBJECT_KEYS
    return false;
#else
    return length > 100;
#endif
}

/**
 * The low bits of every AST word indicate the value's type. This representation
 * is internal and subject to change.
 */
enum class tag : uint8_t {
    integer,
    double_,
    null,
    false_,
    true_,
    string,
    array,
    object,
};

static const size_t TAG_BITS = 3;
static const size_t TAG_MASK = (1 << TAG_BITS) - 1;
static const size_t VALUE_MASK = ~size_t{} >> TAG_BITS;

static const size_t ROOT_MARKER = VALUE_MASK;

constexpr inline tag get_element_tag(size_t s) {
    return static_cast<tag>(s & TAG_MASK);
}

constexpr inline size_t get_element_value(size_t s) { return s >> TAG_BITS; }

constexpr inline size_t make_element(tag t, size_t value) {
    // assert((value & ~VALUE_MASK) == 0);
    // value &= VALUE_MASK;
    return static_cast<size_t>(t) | (value << TAG_BITS);
}

// This template utilizes the One Definition Rule to create global arrays in a
// header. This trick courtesy of Rich Geldreich's Purple JSON parser.
template <typename unused = void>
struct globals_struct {
    static const unsigned char parse_flags[256];
};
typedef globals_struct<> globals;

// clang-format off

    // bit 0 (1) - set if: plain ASCII string character
    // bit 1 (2) - set if: whitespace
    // bit 4 (0x10) - set if: 0-9 e E .
    template <typename unused>
    constexpr uint8_t globals_struct<unused>::parse_flags[256] = {
     // 0    1    2    3    4    5    6    7      8    9    A    B    C    D    E    F
        0,   0,   0,   0,   0,   0,   0,   0,     0,   2,   2,   0,   0,   2,   0,   0, // 0
        0,   0,   0,   0,   0,   0,   0,   0,     0,   0,   0,   0,   0,   0,   0,   0, // 1
        3,   1,   0,   1,   1,   1,   1,   1,     1,   1,   1,   1,   1,   1,   0x11,1, // 2
        0x11,0x11,0x11,0x11,0x11,0x11,0x11,0x11,  0x11,0x11,1,   1,   1,   1,   1,   1, // 3
        1,   1,   1,   1,   1,   0x11,1,   1,     1,   1,   1,   1,   1,   1,   1,   1, // 4
        1,   1,   1,   1,   1,   1,   1,   1,     1,   1,   1,   1,   0,   1,   1,   1, // 5
        1,   1,   1,   1,   1,   0x11,1,   1,     1,   1,   1,   1,   1,   1,   1,   1, // 6
        1,   1,   1,   1,   1,   1,   1,   1,     1,   1,   1,   1,   1,   1,   1,   1, // 7

        // 128-255
        0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0,
        0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0,
        0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0,
        0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0
    };

// clang-format on

constexpr inline bool is_plain_string_character(char c) {
    // return c >= 0x20 && c <= 0x7f && c != 0x22 && c != 0x5c;
    return (globals::parse_flags[static_cast<unsigned char>(c)] & 1) != 0;
}

constexpr inline bool is_whitespace(char c) {
    // return c == '\r' || c == '\n' || c == '\t' || c == ' ';
    return (globals::parse_flags[static_cast<unsigned char>(c)] & 2) != 0;
}

class allocated_buffer {
public:
    allocated_buffer()
        : memory(0) {}

    explicit allocated_buffer(size_t length) {
        // throws std::bad_alloc upon allocation failure
        void* buffer = operator new(sizeof(size_t) + length);
        memory = static_cast<layout*>(buffer);
        memory->refcount = 1;
    }

    allocated_buffer(const allocated_buffer& that)
        : memory(that.memory) {
        incref();
    }

    allocated_buffer(allocated_buffer&& that)
        : memory(that.memory) {
        that.memory = 0;
    }

    ~allocated_buffer() { decref(); }

    allocated_buffer& operator=(const allocated_buffer& that) {
        if (this != &that) {
            decref();
            memory = that.memory;
            incref();
        }
        return *this;
    }

    allocated_buffer& operator=(allocated_buffer&& that) {
        if (this != &that) {
            decref();
            memory = that.memory;
            that.memory = 0;
        }
        return *this;
    }

    char* get_data() const { return memory ? memory->data : 0; }

private:
    void incref() const {
        if (memory) {
            ++(memory->refcount);
        }
    }

    void decref() const {
        if (memory && --(memory->refcount) == 0) {
            operator delete(memory);
        }
    }

    struct layout {
        size_t refcount;
        char data[];
    };

    layout* memory;
};
} // namespace internal

/// A simple type encoding a pointer to some memory and a length (in bytes).
/// Does not maintain any memory.
class string {
public:
    string(const char* text_, size_t length)
        : text(text_)
        , _length(length) {}

    const char* data() const { return text; }

    size_t length() const { return _length; }

#ifndef SAJSON_NO_STD_STRING
    std::string as_string() const { return std::string(text, text + _length); }
#endif

private:
    const char* const text;
    const size_t _length;

    string(); /*=delete*/
};

/// A convenient way to parse JSON from a string literal.  The string ends
/// at its first NUL character.
class literal : public string {
public:
    template <size_t sz>
    explicit literal(const char (&text_)[sz])
        : string(text_, sz - 1) {
        static_assert(sz > 0, "!");
    }
};

/// A pointer to a mutable buffer, its size in bytes, and strong ownership of
/// any copied memory.
class mutable_string_view {
public:
    /// Creates an empty, zero-sized view.
    mutable_string_view()
        : length_(0)
        , data(0)
        , buffer() {}

    /// Given a length in bytes and a pointer, constructs a view
    /// that does not allocate a copy of the data or maintain its life.
    /// The given pointer must stay valid for the duration of the parse and the
    /// resulting \ref document's life.
    mutable_string_view(size_t length, char* data_)
        : length_(length)
        , data(data_)
        , buffer() {}

    /// Allocates a copy of the given \ref literal string and exposes a
    /// mutable view into it.  Throws std::bad_alloc if allocation fails.
    mutable_string_view(const literal& s)
        : length_(s.length())
        , buffer(length_) {
        data = buffer.get_data();
        memcpy(data, s.data(), length_);
    }

    /// Allocates a copy of the given \ref string and exposes a mutable view
    /// into it.  Throws std::bad_alloc if allocation fails.
    mutable_string_view(const string& s)
        : length_(s.length())
        , buffer(length_) {
        data = buffer.get_data();
        memcpy(data, s.data(), length_);
    }

    /// Copies a mutable_string_view.  If any backing memory has been
    /// allocated, its refcount is incremented - both views can safely
    /// use the memory.
    mutable_string_view(const mutable_string_view& that)
        : length_(that.length_)
        , data(that.data)
        , buffer(that.buffer) {}

    /// Move constructor - neuters the old mutable_string_view.
    mutable_string_view(mutable_string_view&& that)
        : length_(that.length_)
        , data(that.data)
        , buffer(std::move(that.buffer)) {
        that.length_ = 0;
        that.data = 0;
    }

    mutable_string_view& operator=(mutable_string_view&& that) {
        if (this != &that) {
            length_ = that.length_;
            data = that.data;
            buffer = std::move(that.buffer);
            that.length_ = 0;
            that.data = 0;
        }
        return *this;
    }

    mutable_string_view& operator=(const mutable_string_view& that) {
        if (this != &that) {
            length_ = that.length_;
            data = that.data;
            buffer = that.buffer;
        }
        return *this;
    }

    size_t length() const { return length_; }

    char* get_data() const { return data; }

private:
    size_t length_;
    char* data;
    internal::allocated_buffer buffer; // may not be allocated
};

namespace internal {
struct object_key_record {
    size_t key_start;
    size_t key_end;
    size_t value;

    bool match(const char* object_data, const string& str) const {
        size_t length = key_end - key_start;
        return length == str.length()
            && 0 == memcmp(str.data(), object_data + key_start, length);
    }
};

struct object_key_comparator {
    object_key_comparator(const char* object_data)
        : data(object_data) {}

    bool operator()(const object_key_record& lhs, const string& rhs) const {
        const size_t lhs_length = lhs.key_end - lhs.key_start;
        const size_t rhs_length = rhs.length();
        if (lhs_length < rhs_length) {
            return true;
        } else if (lhs_length > rhs_length) {
            return false;
        }
        return memcmp(data + lhs.key_start, rhs.data(), lhs_length) < 0;
    }

    bool operator()(const string& lhs, const object_key_record& rhs) const {
        return !(*this)(rhs, lhs);
    }

    bool
    operator()(const object_key_record& lhs, const object_key_record& rhs) {
        const size_t lhs_length = lhs.key_end - lhs.key_start;
        const size_t rhs_length = rhs.key_end - rhs.key_start;
        if (lhs_length < rhs_length) {
            return true;
        } else if (lhs_length > rhs_length) {
            return false;
        }
        return memcmp(data + lhs.key_start, data + rhs.key_start, lhs_length)
            < 0;
    }

    const char* data;
};
} // namespace internal

namespace integer_storage {
enum { word_length = 1 };

inline int load(const size_t* location) {
    int value;
    memcpy(&value, location, sizeof(value));
    return value;
}

inline void store(size_t* location, int value) {
    // NOTE: Most modern compilers optimize away this constant-size
    // memcpy into a single instruction. If any don't, and treat
    // punning through a union as legal, they can be special-cased.
    static_assert(
        sizeof(value) <= sizeof(*location),
        "size_t must not be smaller than int");
    memcpy(location, &value, sizeof(value));
}
} // namespace integer_storage

namespace double_storage {
enum { word_length = sizeof(double) / sizeof(size_t) };

inline double load(const size_t* location) {
    double value;
    memcpy(&value, location, sizeof(double));
    return value;
}

inline void store(size_t* location, double value) {
    // NOTE: Most modern compilers optimize away this constant-size
    // memcpy into a single instruction. If any don't, and treat
    // punning through a union as legal, they can be special-cased.
    memcpy(location, &value, sizeof(double));
}
} // namespace double_storage

/// Represents a JSON value.  First, call get_type() to check its type,
/// which determines which methods are available.
///
/// Note that \ref value does not maintain any backing memory, only the
/// corresponding \ref document does.  It is illegal to access a \ref value
/// after its \ref document has been destroyed.
class value {
public:
    value()
        : value_tag{ tag::null }
        , payload{ nullptr }
        , text{ nullptr } {}

    /// Returns the JSON value's \ref type.
    type get_type() const {
        // As of 2020, current versions of MSVC generate a jump table for this
        // conversion. If it matters, a more clever mapping with knowledge of
        // the specific values is possible. gcc and clang generate good code --
        // at worst a table lookup.
        switch (value_tag) {
        case tag::integer:
            return TYPE_INTEGER;
        case tag::double_:
            return TYPE_DOUBLE;
        case tag::null:
            return TYPE_NULL;
        case tag::false_:
            return TYPE_FALSE;
        case tag::true_:
            return TYPE_TRUE;
        case tag::string:
            return TYPE_STRING;
        case tag::array:
            return TYPE_ARRAY;
        case tag::object:
            return TYPE_OBJECT;
        }
        SAJSON_UNREACHABLE();
    }

    bool is_boolean() const {
        return value_tag == tag::false_ || value_tag == tag::true_;
    }

    bool get_boolean_value() const {
        switch (value_tag) {
        case tag::true_:
            return true;
        case tag::false_:
            return false;
        default:
            assert(false);
            return false;
        }
    }

    /// Returns the length of the object or array.
    /// Only legal if get_type() is TYPE_ARRAY or TYPE_OBJECT.
    size_t get_length() const {
        assert_tag_2(tag::array, tag::object);
        return payload[0];
    }

    /// Returns the nth element of an array.  Calling with an out-of-bound
    /// index is undefined behavior.
    /// Only legal if get_type() is TYPE_ARRAY.
    value get_array_element(size_t index) const {
        using namespace internal;
        assert_tag(tag::array);
        size_t element = payload[1 + index];
        return value(
            get_element_tag(element),
            payload + get_element_value(element),
            text);
    }

    /// Returns the nth key of an object.  Calling with an out-of-bound
    /// index is undefined behavior.
    /// Only legal if get_type() is TYPE_OBJECT.
    string get_object_key(size_t index) const {
        assert_tag(tag::object);
        const size_t* s = payload + 1 + index * 3;
        return string(text + s[0], s[1] - s[0]);
    }

    /// Returns the nth value of an object.  Calling with an out-of-bound
    /// index is undefined behavior.  Only legal if get_type() is TYPE_OBJECT.
    value get_object_value(size_t index) const {
        using namespace internal;
        assert_tag(tag::object);
        size_t element = payload[3 + index * 3];
        return value(
            get_element_tag(element),
            payload + get_element_value(element),
            text);
    }

    /// Given a string key, returns the value with that key or a null value
    /// if the key is not found.  Running time is O(lg N).
    /// Only legal if get_type() is TYPE_OBJECT.
    value get_value_of_key(const string& key) const {
        assert_tag(tag::object);
        size_t i = find_object_key(key);
        if (i < get_length()) {
            return get_object_value(i);
        } else {
            return value(tag::null, 0, 0);
        }
    }

    /// Given a string key, returns the index of the associated value if
    /// one exists.  Returns get_length() if there is no such key.
    /// Note: sajson sorts object keys, so the running time is O(lg N).
    /// Only legal if get_type() is TYPE_OBJECT
    size_t find_object_key(const string& key) const {
        using namespace internal;
        assert_tag(tag::object);
        size_t length = get_length();
        const object_key_record* start
            = reinterpret_cast<const object_key_record*>(payload + 1);
        const object_key_record* end = start + length;
        if (SAJSON_UNLIKELY(should_binary_search(length))) {
            const object_key_record* i = std::lower_bound(
                start, end, key, object_key_comparator(text));
            if (i != end && i->match(text, key)) {
                return i - start;
            }
        } else {
            for (size_t i = 0; i < length; ++i) {
                if (start[i].match(text, key)) {
                    return i;
                }
            }
        }
        return length;
    }

    /// If a numeric value was parsed as a 32-bit integer, returns it.
    /// Only legal if get_type() is TYPE_INTEGER.
    int get_integer_value() const {
        assert_tag(tag::integer);
        return integer_storage::load(payload);
    }

    /// If a numeric value was parsed as a double, returns it.
    /// Only legal if get_type() is TYPE_DOUBLE.
    double get_double_value() const {
        assert_tag(tag::double_);
        return double_storage::load(payload);
    }

    /// Returns a numeric value as a double-precision float.
    /// Only legal if get_type() is TYPE_INTEGER or TYPE_DOUBLE.
    double get_number_value() const {
        assert_tag_2(tag::integer, tag::double_);
        if (value_tag == tag::integer) {
            return get_integer_value();
        } else {
            return get_double_value();
        }
    }

    /// Returns true and writes to the output argument if the numeric value
    /// fits in a 53-bit integer.  This is useful for timestamps and other
    /// situations where integral values with greater than 32-bit precision
    /// are used, as 64-bit values are not understood by all JSON
    /// implementations or languages.
    /// Returns false if the value is not an integer or not in range.
    /// Only legal if get_type() is TYPE_INTEGER or TYPE_DOUBLE.
    bool get_int53_value(int64_t* out) const {
        // Make sure the output variable is always defined to avoid any
        // possible situation like
        // https://gist.github.com/chadaustin/2c249cb850619ddec05b23ca42cf7a18
        *out = 0;

        assert_tag_2(tag::integer, tag::double_);
        switch (value_tag) {
        case tag::integer:
            *out = get_integer_value();
            return true;
        case tag::double_: {
            double v = get_double_value();
            if (v < -(1LL << 53) || v > (1LL << 53)) {
                return false;
            }
            int64_t as_int = static_cast<int64_t>(v);
            if (as_int != v) {
                return false;
            }
            *out = as_int;
            return true;
        }
        default:
            return false;
        }
    }

    /// Returns the length of the string.
    /// Only legal if get_type() is TYPE_STRING.
    size_t get_string_length() const {
        assert_tag(tag::string);
        return payload[1] - payload[0];
    }

    /// Returns a pointer to the beginning of a string value's data.
    /// WARNING: Calling this function and using the return value as a
    /// C-style string (that is, without also using get_string_length())
    /// will cause the string to appear truncated if the string has
    /// embedded NULs.
    /// Only legal if get_type() is TYPE_STRING.
    const char* as_cstring() const {
        assert_tag(tag::string);
        return text + payload[0];
    }

#ifndef SAJSON_NO_STD_STRING
    /// Returns a string's value as a std::string.
    /// Only legal if get_type() is TYPE_STRING.
    std::string as_string() const {
        assert_tag(tag::string);
        return std::string(text + payload[0], text + payload[1]);
    }
#endif

    /// \cond INTERNAL
    const size_t* _internal_get_payload() const { return payload; }
    /// \endcond

private:
    using tag = internal::tag;

    explicit value(tag value_tag_, const size_t* payload_, const char* text_)
        : value_tag(value_tag_)
        , payload(payload_)
        , text(text_) {}

    void assert_tag(tag expected) const { assert(expected == value_tag); }

    void assert_tag_2(tag e1, tag e2) const {
        assert(e1 == value_tag || e2 == value_tag);
    }

    void assert_in_bounds(size_t i) const { assert(i < get_length()); }

    const tag value_tag;
    const size_t* const payload;
    const char* const text;

    friend class document;
};

/// Error code indicating why parse failed.
enum error {
    ERROR_NO_ERROR,
    ERROR_OUT_OF_MEMORY,
    ERROR_UNEXPECTED_END,
    ERROR_MISSING_ROOT_ELEMENT,
    ERROR_BAD_ROOT,
    ERROR_EXPECTED_COMMA,
    ERROR_MISSING_OBJECT_KEY,
    ERROR_EXPECTED_COLON,
    ERROR_EXPECTED_END_OF_INPUT,
    ERROR_UNEXPECTED_COMMA,
    ERROR_EXPECTED_VALUE,
    ERROR_EXPECTED_NULL,
    ERROR_EXPECTED_FALSE,
    ERROR_EXPECTED_TRUE,
    ERROR_INVALID_NUMBER,
    ERROR_MISSING_EXPONENT,
    ERROR_ILLEGAL_CODEPOINT,
    ERROR_INVALID_UNICODE_ESCAPE,
    ERROR_UNEXPECTED_END_OF_UTF16,
    ERROR_EXPECTED_U,
    ERROR_INVALID_UTF16_TRAIL_SURROGATE,
    ERROR_UNKNOWN_ESCAPE,
    ERROR_INVALID_UTF8,
    ERROR_UNINITIALIZED,
};

namespace internal {
class ownership {
public:
    ownership() = delete;
    ownership(const ownership&) = delete;
    void operator=(const ownership&) = delete;

    explicit ownership(size_t* p_)
        : p(p_) {}

    ownership(ownership&& p_)
        : p(p_.p) {
        p_.p = 0;
    }

    ~ownership() { delete[] p; }

    bool is_valid() const { return !!p; }

private:
    size_t* p;
};

inline const char* get_error_text(error error_code) {
    switch (error_code) {
    case ERROR_NO_ERROR:
        return "no error";
    case ERROR_OUT_OF_MEMORY:
        return "out of memory";
    case ERROR_UNEXPECTED_END:
        return "unexpected end of input";
    case ERROR_MISSING_ROOT_ELEMENT:
        return "missing root element";
    case ERROR_BAD_ROOT:
        return "document root must be object or array";
    case ERROR_EXPECTED_COMMA:
        return "expected ,";
    case ERROR_MISSING_OBJECT_KEY:
        return "missing object key";
    case ERROR_EXPECTED_COLON:
        return "expected :";
    case ERROR_EXPECTED_END_OF_INPUT:
        return "expected end of input";
    case ERROR_UNEXPECTED_COMMA:
        return "unexpected comma";
    case ERROR_EXPECTED_VALUE:
        return "expected value";
    case ERROR_EXPECTED_NULL:
        return "expected 'null'";
    case ERROR_EXPECTED_FALSE:
        return "expected 'false'";
    case ERROR_EXPECTED_TRUE:
        return "expected 'true'";
    case ERROR_INVALID_NUMBER:
        return "invalid number";
    case ERROR_MISSING_EXPONENT:
        return "missing exponent";
    case ERROR_ILLEGAL_CODEPOINT:
        return "illegal unprintable codepoint in string";
    case ERROR_INVALID_UNICODE_ESCAPE:
        return "invalid character in unicode escape";
    case ERROR_UNEXPECTED_END_OF_UTF16:
        return "unexpected end of input during UTF-16 surrogate pair";
    case ERROR_EXPECTED_U:
        return "expected \\u";
    case ERROR_INVALID_UTF16_TRAIL_SURROGATE:
        return "invalid UTF-16 trail surrogate";
    case ERROR_UNKNOWN_ESCAPE:
        return "unknown escape";
    case ERROR_INVALID_UTF8:
        return "invalid UTF-8";
    case ERROR_UNINITIALIZED:
        return "uninitialized document";
    }

    SAJSON_UNREACHABLE();
}
} // namespace internal

/**
 * Represents the result of a JSON parse: either is_valid() and the document
 * contains a root value or parse error information is available.
 *
 * Note that the document holds a strong reference to any memory allocated:
 * any mutable copy of the input text and any memory allocated for the
 * AST data structure.  Thus, the document must not be deallocated while any
 * \ref value is in use.
 */
class document {
public:
    document()
        : document{ mutable_string_view{}, 0, 0, ERROR_UNINITIALIZED, 0 } {}

    document(document&& rhs)
        : input(rhs.input)
        , structure(std::move(rhs.structure))
        , root_tag(rhs.root_tag)
        , root(rhs.root)
        , error_line(rhs.error_line)
        , error_column(rhs.error_column)
        , error_code(rhs.error_code)
        , error_arg(rhs.error_arg) {
        // Yikes... but strcpy is okay here because formatted_error is
        // guaranteed to be null-terminated.
        strcpy(formatted_error_message, rhs.formatted_error_message);
        // should rhs's fields be zeroed too?
    }

    /**
     * Returns true if the document was parsed successfully.
     * If true, call get_root() to access the document's root value.
     * If false, call get_error_line(), get_error_column(), and
     * get_error_message_as_cstring() to see why the parse failed.
     */
    bool is_valid() const {
        return root_tag == tag::array || root_tag == tag::object;
    }

    /// If is_valid(), returns the document's root \ref value.
    value get_root() const { return value(root_tag, root, input.get_data()); }

    /// If not is_valid(), returns the one-based line number where the parse
    /// failed.
    size_t get_error_line() const { return error_line; }

    /// If not is_valid(), returns the one-based column number where the parse
    /// failed.
    size_t get_error_column() const { return error_column; }

#ifndef SAJSON_NO_STD_STRING
    /// If not is_valid(), returns a std::string indicating why the parse
    /// failed.
    std::string get_error_message_as_string() const {
        return formatted_error_message;
    }
#endif

    /// If not is_valid(), returns a null-terminated C string indicating why the
    /// parse failed.
    const char* get_error_message_as_cstring() const {
        return formatted_error_message;
    }

    /// \cond INTERNAL

    // WARNING: Internal function which is subject to change
    error _internal_get_error_code() const { return error_code; }

    // WARNING: Internal function which is subject to change
    int _internal_get_error_argument() const { return error_arg; }

    // WARNING: Internal function which is subject to change
    const char* _internal_get_error_text() const {
        return internal::get_error_text(error_code);
    }

    // WARNING: Internal function exposed only for high-performance language
    // bindings.
    internal::tag _internal_get_root_tag() const { return root_tag; }

    // WARNING: Internal function exposed only for high-performance language
    // bindings.
    const size_t* _internal_get_root() const { return root; }

    // WARNING: Internal function exposed only for high-performance language
    // bindings.
    const mutable_string_view& _internal_get_input() const { return input; }

    /// \endcond

private:
    using tag = internal::tag;

    document(const document&) = delete;
    void operator=(const document&) = delete;

    explicit document(
        const mutable_string_view& input_,
        internal::ownership&& structure_,
        tag root_tag_,
        const size_t* root_)
        : input(input_)
        , structure(std::move(structure_))
        , root_tag(root_tag_)
        , root(root_)
        , error_line(0)
        , error_column(0)
        , error_code(ERROR_NO_ERROR)
        , error_arg(0) {
        formatted_error_message[0] = 0;
    }

    explicit document(
        const mutable_string_view& input_,
        size_t error_line_,
        size_t error_column_,
        const error error_code_,
        int error_arg_)
        : input(input_)
        , structure(0)
        , root_tag(tag::null)
        , root(0)
        , error_line(error_line_)
        , error_column(error_column_)
        , error_code(error_code_)
        , error_arg(error_arg_) {
        formatted_error_message[ERROR_BUFFER_LENGTH - 1] = 0;
        int written = has_significant_error_arg()
            ? SAJSON_snprintf(
                  formatted_error_message,
                  ERROR_BUFFER_LENGTH - 1,
                  "%s: %d",
                  _internal_get_error_text(),
                  error_arg)
            : SAJSON_snprintf(
                  formatted_error_message,
                  ERROR_BUFFER_LENGTH - 1,
                  "%s",
                  _internal_get_error_text());
        (void)written;
        assert(written >= 0 && written < ERROR_BUFFER_LENGTH);
    }

    bool has_significant_error_arg() const {
        return error_code == ERROR_ILLEGAL_CODEPOINT;
    }

    mutable_string_view input;
    internal::ownership structure;
    const tag root_tag;
    const size_t* const root;
    const size_t error_line;
    const size_t error_column;
    const error error_code;
    const int error_arg;

    enum { ERROR_BUFFER_LENGTH = 128 };
    char formatted_error_message[ERROR_BUFFER_LENGTH];

    template <typename AllocationStrategy, typename StringType>
    friend document
    parse(const AllocationStrategy& strategy, const StringType& string);
    template <typename Allocator>
    friend class parser;
};

/// Allocation policy that allocates one large buffer guaranteed to hold the
/// resulting AST.  This allocation policy is the fastest since it requires
/// no conditionals to see if more memory must be allocated.
class single_allocation {
public:
    /// \cond INTERNAL

    class stack_head {
    public:
        stack_head(stack_head&& other)
            : stack_bottom(other.stack_bottom)
            , stack_top(other.stack_top) {}

        bool push(size_t element) {
            *stack_top++ = element;
            return true;
        }

        size_t* reserve(size_t amount, bool* success) {
            size_t* rv = stack_top;
            stack_top += amount;
            *success = true;
            return rv;
        }

        // The compiler does not see the stack_head (stored in a local)
        // and the allocator (stored as a field) have the same stack_bottom
        // values, so it does a bit of redundant work.
        // So there's a microoptimization available here: introduce a type
        // "stack_mark" and make it polymorphic on the allocator.  For
        // single_allocation, it merely needs to be a single pointer.

        void reset(size_t new_top) { stack_top = stack_bottom + new_top; }

        size_t get_size() { return stack_top - stack_bottom; }

        size_t* get_top() { return stack_top; }

        size_t* get_pointer_from_offset(size_t offset) {
            return stack_bottom + offset;
        }

    private:
        stack_head() = delete;
        stack_head(const stack_head&) = delete;
        void operator=(const stack_head&) = delete;

        explicit stack_head(size_t* base)
            : stack_bottom(base)
            , stack_top(base) {}

        size_t* const stack_bottom;
        size_t* stack_top;

        friend class single_allocation;
    };

    class allocator {
    public:
        allocator() = delete;
        allocator(const allocator&) = delete;
        void operator=(const allocator&) = delete;

        explicit allocator(
            size_t* buffer, size_t input_size, bool should_deallocate_)
            : structure(buffer)
            , structure_end(buffer ? buffer + input_size : 0)
            , write_cursor(structure_end)
            , should_deallocate(should_deallocate_) {}

        explicit allocator(std::nullptr_t)
            : structure(0)
            , structure_end(0)
            , write_cursor(0)
            , should_deallocate(false) {}

        allocator(allocator&& other)
            : structure(other.structure)
            , structure_end(other.structure_end)
            , write_cursor(other.write_cursor)
            , should_deallocate(other.should_deallocate) {
            other.structure = 0;
            other.structure_end = 0;
            other.write_cursor = 0;
            other.should_deallocate = false;
        }

        ~allocator() {
            if (should_deallocate) {
                delete[] structure;
            }
        }

        stack_head get_stack_head(bool* success) {
            *success = true;
            return stack_head(structure);
        }

        size_t get_write_offset() { return structure_end - write_cursor; }

        size_t* get_write_pointer_of(size_t v) { return structure_end - v; }

        size_t* reserve(size_t size, bool* success) {
            *success = true;
            write_cursor -= size;
            return write_cursor;
        }

        size_t* get_ast_root() { return write_cursor; }

        internal::ownership transfer_ownership() {
            auto p = structure;
            structure = 0;
            structure_end = 0;
            write_cursor = 0;
            if (should_deallocate) {
                return internal::ownership(p);
            } else {
                return internal::ownership(0);
            }
        }

    private:
        size_t* structure;
        size_t* structure_end;
        size_t* write_cursor;
        bool should_deallocate;
    };

    /// \endcond

    /// Allocate a single worst-case AST buffer with one word per byte in
    /// the input document.
    single_allocation()
        : has_existing_buffer(false)
        , existing_buffer(0)
        , existing_buffer_size(0) {}

    /// Write the AST into an existing buffer.  Will fail with an out of
    /// memory error if the buffer is not guaranteed to be big enough for
    /// the document.  The caller must guarantee the memory is valid for
    /// the duration of the parse and the AST traversal.
    single_allocation(size_t* existing_buffer_, size_t size_in_words)
        : has_existing_buffer(true)
        , existing_buffer(existing_buffer_)
        , existing_buffer_size(size_in_words) {}

    /// Convenience wrapper for single_allocation(size_t*, size_t) that
    /// automatically infers the length of a given array.
    template <size_t N>
    explicit single_allocation(size_t (&existing_buffer_)[N])
        : single_allocation(existing_buffer_, N) {}

    /// \cond INTERNAL

    allocator
    make_allocator(size_t input_document_size_in_bytes, bool* succeeded) const {
        if (has_existing_buffer) {
            if (existing_buffer_size < input_document_size_in_bytes) {
                *succeeded = false;
                return allocator(nullptr);
            }
            *succeeded = true;
            return allocator(
                existing_buffer, input_document_size_in_bytes, false);
        } else {
            size_t* buffer
                = new (std::nothrow) size_t[input_document_size_in_bytes];
            if (!buffer) {
                *succeeded = false;
                return allocator(nullptr);
            }
            *succeeded = true;
            return allocator(buffer, input_document_size_in_bytes, true);
        }
    }

    /// \endcond

private:
    bool has_existing_buffer;
    size_t* existing_buffer;
    size_t existing_buffer_size;
};

/// Allocation policy that uses dynamically-growing buffers for both the
/// parse stack and the AST.  This allocation policy minimizes peak memory
/// usage at the cost of some allocation and copying churn.
class dynamic_allocation {
public:
    /// \cond INTERNAL

    class stack_head {
    public:
        stack_head(stack_head&& other)
            : stack_top(other.stack_top)
            , stack_bottom(other.stack_bottom)
            , stack_limit(other.stack_limit) {
            other.stack_top = 0;
            other.stack_bottom = 0;
            other.stack_limit = 0;
        }

        ~stack_head() { delete[] stack_bottom; }

        bool push(size_t element) {
            if (can_grow(1)) {
                *stack_top++ = element;
                return true;
            } else {
                return false;
            }
        }

        size_t* reserve(size_t amount, bool* success) {
            if (can_grow(amount)) {
                size_t* rv = stack_top;
                stack_top += amount;
                *success = true;
                return rv;
            } else {
                *success = false;
                return 0;
            }
        }

        void reset(size_t new_top) { stack_top = stack_bottom + new_top; }

        size_t get_size() { return stack_top - stack_bottom; }

        size_t* get_top() { return stack_top; }

        size_t* get_pointer_from_offset(size_t offset) {
            return stack_bottom + offset;
        }

    private:
        stack_head(const stack_head&) = delete;
        void operator=(const stack_head&) = delete;

        explicit stack_head(size_t initial_capacity, bool* success) {
            assert(initial_capacity);
            stack_bottom = new (std::nothrow) size_t[initial_capacity];
            stack_top = stack_bottom;
            if (stack_bottom) {
                stack_limit = stack_bottom + initial_capacity;
            } else {
                stack_limit = 0;
            }
            *success = !!stack_bottom;
        }

        bool can_grow(size_t amount) {
            if (SAJSON_LIKELY(
                    amount <= static_cast<size_t>(stack_limit - stack_top))) {
                return true;
            }

            size_t current_size = stack_top - stack_bottom;
            size_t old_capacity = stack_limit - stack_bottom;
            size_t new_capacity = old_capacity * 2;
            while (new_capacity < amount + current_size) {
                new_capacity *= 2;
            }
            size_t* new_stack = new (std::nothrow) size_t[new_capacity];
            if (!new_stack) {
                stack_top = 0;
                stack_bottom = 0;
                stack_limit = 0;
                return false;
            }

            memcpy(new_stack, stack_bottom, current_size * sizeof(size_t));
            delete[] stack_bottom;
            stack_top = new_stack + current_size;
            stack_bottom = new_stack;
            stack_limit = stack_bottom + new_capacity;
            return true;
        }

        size_t* stack_top; // stack grows up: stack_top >= stack_bottom
        size_t* stack_bottom;
        size_t* stack_limit;

        friend class dynamic_allocation;
    };

    class allocator {
    public:
        allocator() = delete;
        allocator(const allocator&) = delete;
        void operator=(const allocator&) = delete;

        explicit allocator(
            size_t* buffer_,
            size_t current_capacity,
            size_t initial_stack_capacity_)
            : ast_buffer_bottom(buffer_)
            , ast_buffer_top(buffer_ + current_capacity)
            , ast_write_head(ast_buffer_top)
            , initial_stack_capacity(initial_stack_capacity_) {}

        explicit allocator(std::nullptr_t)
            : ast_buffer_bottom(0)
            , ast_buffer_top(0)
            , ast_write_head(0)
            , initial_stack_capacity(0) {}

        allocator(allocator&& other)
            : ast_buffer_bottom(other.ast_buffer_bottom)
            , ast_buffer_top(other.ast_buffer_top)
            , ast_write_head(other.ast_write_head)
            , initial_stack_capacity(other.initial_stack_capacity) {
            other.ast_buffer_bottom = 0;
            other.ast_buffer_top = 0;
            other.ast_write_head = 0;
        }

        ~allocator() { delete[] ast_buffer_bottom; }

        stack_head get_stack_head(bool* success) {
            return stack_head(initial_stack_capacity, success);
        }

        size_t get_write_offset() { return ast_buffer_top - ast_write_head; }

        size_t* get_write_pointer_of(size_t v) { return ast_buffer_top - v; }

        size_t* reserve(size_t size, bool* success) {
            if (can_grow(size)) {
                ast_write_head -= size;
                *success = true;
                return ast_write_head;
            } else {
                *success = false;
                return 0;
            }
        }

        size_t* get_ast_root() { return ast_write_head; }

        internal::ownership transfer_ownership() {
            auto p = ast_buffer_bottom;
            ast_buffer_bottom = 0;
            ast_buffer_top = 0;
            ast_write_head = 0;
            return internal::ownership(p);
        }

    private:
        bool can_grow(size_t amount) {
            if (SAJSON_LIKELY(
                    amount <= static_cast<size_t>(
                                  ast_write_head - ast_buffer_bottom))) {
                return true;
            }
            size_t current_capacity = ast_buffer_top - ast_buffer_bottom;

            size_t current_size = ast_buffer_top - ast_write_head;
            size_t new_capacity = current_capacity * 2;
            while (new_capacity < amount + current_size) {
                new_capacity *= 2;
            }

            size_t* old_buffer = ast_buffer_bottom;
            size_t* new_buffer = new (std::nothrow) size_t[new_capacity];
            if (!new_buffer) {
                ast_buffer_bottom = 0;
                ast_buffer_top = 0;
                ast_write_head = 0;
                return false;
            }

            size_t* old_write_head = ast_write_head;
            ast_buffer_bottom = new_buffer;
            ast_buffer_top = new_buffer + new_capacity;
            ast_write_head = ast_buffer_top - current_size;
            memcpy(
                ast_write_head, old_write_head, current_size * sizeof(size_t));
            delete[] old_buffer;

            return true;
        }

        size_t*
            ast_buffer_bottom; // base address of the ast buffer - it grows down
        size_t* ast_buffer_top;
        size_t* ast_write_head;
        size_t initial_stack_capacity;
    };

    /// \endcond

    /// Creates a dynamic_allocation policy with the given initial AST
    /// and stack buffer sizes.
    dynamic_allocation(
        size_t initial_ast_capacity_ = 0, size_t initial_stack_capacity_ = 0)
        : initial_ast_capacity(initial_ast_capacity_)
        , initial_stack_capacity(initial_stack_capacity_) {}

    /// \cond INTERNAL

    allocator
    make_allocator(size_t input_document_size_in_bytes, bool* succeeded) const {
        size_t capacity = initial_ast_capacity;
        if (!capacity) {
            // TODO: guess based on input document size
            capacity = 1024;
        }

        size_t* buffer = new (std::nothrow) size_t[capacity];
        if (!buffer) {
            *succeeded = false;
            return allocator(nullptr);
        }

        size_t stack_capacity = initial_stack_capacity;
        if (!stack_capacity) {
            stack_capacity = 256;
        }

        *succeeded = true;
        return allocator(buffer, capacity, stack_capacity);
    }

    /// \endcond

private:
    size_t initial_ast_capacity;
    size_t initial_stack_capacity;
};

/// Allocation policy that attempts to fit the parsed AST into an existing
/// memory buffer.  This allocation policy is useful when using sajson in
/// a zero-allocation context or when there are constraints on the amount
// of memory that can be used.
class bounded_allocation {
public:
    /// \cond INTERNAL

    class allocator;

    class stack_head {
    public:
        stack_head(stack_head&& other)
            : source_allocator(other.source_allocator) {
            other.source_allocator = 0;
        }

        bool push(size_t element) {
            if (SAJSON_LIKELY(source_allocator->can_grow(1))) {
                *(source_allocator->stack_top)++ = element;
                return true;
            } else {
                return false;
            }
        }

        size_t* reserve(size_t amount, bool* success) {
            if (SAJSON_LIKELY(source_allocator->can_grow(amount))) {
                size_t* rv = source_allocator->stack_top;
                source_allocator->stack_top += amount;
                *success = true;
                return rv;
            } else {
                *success = false;
                return 0;
            }
        }

        void reset(size_t new_top) {
            source_allocator->stack_top = source_allocator->structure + new_top;
        }

        size_t get_size() {
            return source_allocator->stack_top - source_allocator->structure;
        }

        size_t* get_top() { return source_allocator->stack_top; }

        size_t* get_pointer_from_offset(size_t offset) {
            return source_allocator->structure + offset;
        }

    private:
        stack_head(const stack_head&) = delete;
        void operator=(const stack_head&) = delete;

        explicit stack_head(allocator* source_allocator_)
            : source_allocator(source_allocator_) {}

        allocator* source_allocator;

        friend class bounded_allocation;
    };

    class allocator {
    public:
        allocator() = delete;
        allocator(const allocator&) = delete;
        void operator=(const allocator&) = delete;

        explicit allocator(size_t* existing_buffer, size_t existing_buffer_size)
            : structure(existing_buffer)
            , structure_end(existing_buffer + existing_buffer_size)
            , write_cursor(structure_end)
            , stack_top(structure) {}

        allocator(allocator&& other)
            : structure(other.structure)
            , structure_end(other.structure_end)
            , write_cursor(other.write_cursor)
            , stack_top(other.stack_top) {
            other.structure = 0;
            other.structure_end = 0;
            other.write_cursor = 0;
            other.stack_top = 0;
        }

        stack_head get_stack_head(bool* success) {
            *success = true;
            return stack_head(this);
        }

        size_t get_write_offset() { return structure_end - write_cursor; }

        size_t* get_write_pointer_of(size_t v) { return structure_end - v; }

        size_t* reserve(size_t size, bool* success) {
            if (can_grow(size)) {
                write_cursor -= size;
                *success = true;
                return write_cursor;
            } else {
                *success = false;
                return 0;
            }
        }

        size_t* get_ast_root() { return write_cursor; }

        internal::ownership transfer_ownership() {
            structure = 0;
            structure_end = 0;
            write_cursor = 0;
            return internal::ownership(0);
        }

    private:
        bool can_grow(size_t amount) {
            // invariant: stack_top <= write_cursor
            // thus: write_cursor - stack_top is positive
            return static_cast<size_t>(write_cursor - stack_top) >= amount;
        }

        size_t* structure;
        size_t* structure_end;
        size_t* write_cursor;
        size_t* stack_top;

        friend class bounded_allocation;
    };

    /// \endcond

    /// Uses an existing buffer to hold the parsed AST, if it fits.  The
    /// specified buffer must not be deallocated until after the document
    /// is parsed and the AST traversed.
    bounded_allocation(size_t* existing_buffer_, size_t size_in_words)
        : existing_buffer(existing_buffer_)
        , existing_buffer_size(size_in_words) {}

    /// Convenience wrapper for bounded_allocation(size_t*, size) that
    /// automatically infers the size of the given array.
    template <size_t N>
    explicit bounded_allocation(size_t (&existing_buffer_)[N])
        : bounded_allocation(existing_buffer_, N) {}

    /// \cond INTERNAL

    allocator
    make_allocator(size_t input_document_size_in_bytes, bool* succeeded) const {
        *succeeded = true;
        return allocator(existing_buffer, existing_buffer_size);
    }

    /// \endcond

private:
    size_t* existing_buffer;
    size_t existing_buffer_size;
};

// I thought about putting parser in the internal namespace but I don't
// want to indent it further...
/// \cond INTERNAL
template <typename Allocator>
class parser {
public:
    parser(const mutable_string_view& msv, Allocator&& allocator_)
        : input(msv)
        , input_end(input.get_data() + input.length())
        , allocator(std::move(allocator_))
        , root_tag(internal::tag::null)
        , error_line(0)
        , error_column(0) {}

    document get_document() {
        if (parse()) {
            size_t* ast_root = allocator.get_ast_root();
            return document(
                input, allocator.transfer_ownership(), root_tag, ast_root);
        } else {
            return document(
                input, error_line, error_column, error_code, error_arg);
        }
    }

private:
    struct error_result {
        operator bool() const { return false; }
        operator char*() const { return 0; }
    };

    bool at_eof(const char* p) { return p == input_end; }

    char* skip_whitespace(char* p) {
        // There is an opportunity to make better use of superscalar
        // hardware here* but if someone cares about JSON parsing
        // performance the first thing they do is minify, so prefer
        // to optimize for code size here.
        // *
        // https://github.com/chadaustin/Web-Benchmarks/blob/master/json/third-party/pjson/pjson.h#L1873
        for (;;) {
            if (SAJSON_UNLIKELY(p == input_end)) {
                return 0;
            } else if (internal::is_whitespace(*p)) {
                ++p;
            } else {
                return p;
            }
        }
    }

    error_result oom(char* p, const char* /*reason*/) {
        return make_error(p, ERROR_OUT_OF_MEMORY);
    }

    error_result unexpected_end() {
        return make_error(0, ERROR_UNEXPECTED_END);
    }

    error_result unexpected_end(char* p) {
        return make_error(p, ERROR_UNEXPECTED_END);
    }

    error_result make_error(char* p, error code, int arg = 0) {
        if (!p) {
            p = input_end;
        }

        error_line = 1;
        error_column = 1;

        char* c = input.get_data();
        while (c < p) {
            if (*c == '\r') {
                if (c + 1 < p && c[1] == '\n') {
                    ++error_line;
                    error_column = 1;
                    ++c;
                } else {
                    ++error_line;
                    error_column = 1;
                }
            } else if (*c == '\n') {
                ++error_line;
                error_column = 1;
            } else {
                // TODO: count UTF-8 characters
                ++error_column;
            }
            ++c;
        }

        error_code = code;
        error_arg = arg;
        return error_result();
    }

    bool parse() {
        using namespace internal;

        // p points to the character currently being parsed
        char* p = input.get_data();

        bool success;
        auto stack = allocator.get_stack_head(&success);
        if (SAJSON_UNLIKELY(!success)) {
            return oom(p, "failed to get stack head");
        }

        p = skip_whitespace(p);
        if (SAJSON_UNLIKELY(!p)) {
            return make_error(p, ERROR_MISSING_ROOT_ELEMENT);
        }

        // current_base is an offset to the first element of the current
        // structure (object or array)
        size_t current_base = stack.get_size();
        tag current_structure_tag;
        if (*p == '[') {
            current_structure_tag = tag::array;
            bool s
                = stack.push(make_element(current_structure_tag, ROOT_MARKER));
            if (SAJSON_UNLIKELY(!s)) {
                return oom(p, "stack.push array");
            }
            goto array_close_or_element;
        } else if (*p == '{') {
            current_structure_tag = tag::object;
            bool s
                = stack.push(make_element(current_structure_tag, ROOT_MARKER));
            if (SAJSON_UNLIKELY(!s)) {
                printf("oom 3\n");
                return oom(p, "stack.push object");
            }
            goto object_close_or_element;
        } else {
            return make_error(p, ERROR_BAD_ROOT);
        }

        // BEGIN STATE MACHINE

        size_t pop_element; // used as an argument into the `pop` routine

        if (0) { // purely for structure

        // ASSUMES: byte at p SHOULD be skipped
        array_close_or_element:
            p = skip_whitespace(p + 1);
            if (SAJSON_UNLIKELY(!p)) {
                return unexpected_end();
            }
            if (*p == ']') {
                goto pop_array;
            } else {
                goto next_element;
            }
            SAJSON_UNREACHABLE();

        // ASSUMES: byte at p SHOULD be skipped
        object_close_or_element:
            p = skip_whitespace(p + 1);
            if (SAJSON_UNLIKELY(!p)) {
                return unexpected_end();
            }
            if (*p == '}') {
                goto pop_object;
            } else {
                goto object_key;
            }
            SAJSON_UNREACHABLE();

        // ASSUMES: byte at p SHOULD NOT be skipped
        structure_close_or_comma:
            p = skip_whitespace(p);
            if (SAJSON_UNLIKELY(!p)) {
                return unexpected_end();
            }

            if (current_structure_tag == tag::array) {
                if (*p == ']') {
                    goto pop_array;
                } else {
                    if (SAJSON_UNLIKELY(*p != ',')) {
                        return make_error(p, ERROR_EXPECTED_COMMA);
                    }
                    ++p;
                    goto next_element;
                }
            } else {
                assert(current_structure_tag == tag::object);
                if (*p == '}') {
                    goto pop_object;
                } else {
                    if (SAJSON_UNLIKELY(*p != ',')) {
                        return make_error(p, ERROR_EXPECTED_COMMA);
                    }
                    ++p;
                    goto object_key;
                }
            }
            SAJSON_UNREACHABLE();

        // ASSUMES: *p == '}'
        pop_object : {
            ++p;
            size_t* base_ptr = stack.get_pointer_from_offset(current_base);
            pop_element = *base_ptr;
            if (SAJSON_UNLIKELY(
                    !install_object(base_ptr + 1, stack.get_top()))) {
                return oom(p, "install_object");
            }
            goto pop;
        }

        // ASSUMES: *p == ']'
        pop_array : {
            ++p;
            size_t* base_ptr = stack.get_pointer_from_offset(current_base);
            pop_element = *base_ptr;
            if (SAJSON_UNLIKELY(
                    !install_array(base_ptr + 1, stack.get_top()))) {
                return oom(p, "install_array");
            }
            goto pop;
        }

        // ASSUMES: byte at p SHOULD NOT be skipped
        object_key : {
            p = skip_whitespace(p);
            if (SAJSON_UNLIKELY(!p)) {
                return unexpected_end();
            }
            if (SAJSON_UNLIKELY(*p != '"')) {
                return make_error(p, ERROR_MISSING_OBJECT_KEY);
            }
            bool success_;
            size_t* out = stack.reserve(2, &success_);
            if (SAJSON_UNLIKELY(!success_)) {
                return oom(p, "reserve for object key");
            }
            p = parse_string(p, out);
            if (SAJSON_UNLIKELY(!p)) {
                return false;
            }
            p = skip_whitespace(p);
            if (SAJSON_UNLIKELY(!p || *p != ':')) {
                return make_error(p, ERROR_EXPECTED_COLON);
            }
            ++p;
            goto next_element;
        }

        // ASSUMES: byte at p SHOULD NOT be skipped
        next_element:
            p = skip_whitespace(p);
            if (SAJSON_UNLIKELY(!p)) {
                return unexpected_end();
            }

            tag value_tag_result;
            switch (*p) {
            case 0:
                return unexpected_end(p);
            case 'n':
                p = parse_null(p);
                if (!p) {
                    return false;
                }
                value_tag_result = tag::null;
                break;
            case 'f':
                p = parse_false(p);
                if (!p) {
                    return false;
                }
                value_tag_result = tag::false_;
                break;
            case 't':
                p = parse_true(p);
                if (!p) {
                    return false;
                }
                value_tag_result = tag::true_;
                break;
            case '0':
            case '1':
            case '2':
            case '3':
            case '4':
            case '5':
            case '6':
            case '7':
            case '8':
            case '9':
            case '-': {
                auto result = parse_number(p);
                p = result.first;
                if (!p) {
                    return false;
                }
                value_tag_result = result.second;
                break;
            }
            case '"': {
                bool success_;
                size_t* string_tag = allocator.reserve(2, &success_);
                if (SAJSON_UNLIKELY(!success_)) {
                    return oom(p, "reserve for string tag");
                }
                p = parse_string(p, string_tag);
                if (!p) {
                    return false;
                }
                value_tag_result = tag::string;
                break;
            }

            case '[': {
                size_t previous_base = current_base;
                current_base = stack.get_size();
                bool s = stack.push(
                    make_element(current_structure_tag, previous_base));
                if (SAJSON_UNLIKELY(!s)) {
                    return oom(p, "stack.push array");
                }
                current_structure_tag = tag::array;
                goto array_close_or_element;
            }
            case '{': {
                size_t previous_base = current_base;
                current_base = stack.get_size();
                bool s = stack.push(
                    make_element(current_structure_tag, previous_base));
                if (SAJSON_UNLIKELY(!s)) {
                    return oom(p, "stack.push object");
                }
                current_structure_tag = tag::object;
                goto object_close_or_element;
            }
            pop : {
                size_t parent = get_element_value(pop_element);
                if (parent == ROOT_MARKER) {
                    root_tag = current_structure_tag;
                    p = skip_whitespace(p);
                    if (SAJSON_UNLIKELY(p)) {
                        return make_error(p, ERROR_EXPECTED_END_OF_INPUT);
                    }
                    return true;
                }
                stack.reset(current_base);
                current_base = parent;
                value_tag_result = current_structure_tag;
                current_structure_tag = get_element_tag(pop_element);
                break;
            }

            case ',':
                return make_error(p, ERROR_UNEXPECTED_COMMA);
            default:
                return make_error(p, ERROR_EXPECTED_VALUE);
            }

            bool s = stack.push(
                make_element(value_tag_result, allocator.get_write_offset()));
            if (SAJSON_UNLIKELY(!s)) {
                return oom(p, "stack.push value");
            }

            goto structure_close_or_comma;
        }

        SAJSON_UNREACHABLE();
    }

    bool has_remaining_characters(char* p, ptrdiff_t remaining) {
        return input_end - p >= remaining;
    }

    char* parse_null(char* p) {
        if (SAJSON_UNLIKELY(!has_remaining_characters(p, 4))) {
            make_error(p, ERROR_UNEXPECTED_END);
            return 0;
        }
        char p1 = p[1];
        char p2 = p[2];
        char p3 = p[3];
        if (SAJSON_UNLIKELY(p1 != 'u' || p2 != 'l' || p3 != 'l')) {
            make_error(p, ERROR_EXPECTED_NULL);
            return 0;
        }
        return p + 4;
    }

    char* parse_false(char* p) {
        if (SAJSON_UNLIKELY(!has_remaining_characters(p, 5))) {
            return make_error(p, ERROR_UNEXPECTED_END);
        }
        char p1 = p[1];
        char p2 = p[2];
        char p3 = p[3];
        char p4 = p[4];
        if (SAJSON_UNLIKELY(p1 != 'a' || p2 != 'l' || p3 != 's' || p4 != 'e')) {
            return make_error(p, ERROR_EXPECTED_FALSE);
        }
        return p + 5;
    }

    char* parse_true(char* p) {
        if (SAJSON_UNLIKELY(!has_remaining_characters(p, 4))) {
            return make_error(p, ERROR_UNEXPECTED_END);
        }
        char p1 = p[1];
        char p2 = p[2];
        char p3 = p[3];
        if (SAJSON_UNLIKELY(p1 != 'r' || p2 != 'u' || p3 != 'e')) {
            return make_error(p, ERROR_EXPECTED_TRUE);
        }
        return p + 4;
    }

    static double pow10(int64_t exponent) {
        if (SAJSON_UNLIKELY(exponent > 308)) {
            return std::numeric_limits<double>::infinity();
        } else if (SAJSON_UNLIKELY(exponent < -323)) {
            return 0.0;
        }

        // clang-format off
        static const double constants[] = {
            1e-323,1e-322,1e-321,1e-320,1e-319,1e-318,1e-317,1e-316,1e-315,1e-314,
            1e-313,1e-312,1e-311,1e-310,1e-309,1e-308,1e-307,1e-306,1e-305,1e-304,
            1e-303,1e-302,1e-301,1e-300,1e-299,1e-298,1e-297,1e-296,1e-295,1e-294,
            1e-293,1e-292,1e-291,1e-290,1e-289,1e-288,1e-287,1e-286,1e-285,1e-284,
            1e-283,1e-282,1e-281,1e-280,1e-279,1e-278,1e-277,1e-276,1e-275,1e-274,
            1e-273,1e-272,1e-271,1e-270,1e-269,1e-268,1e-267,1e-266,1e-265,1e-264,
            1e-263,1e-262,1e-261,1e-260,1e-259,1e-258,1e-257,1e-256,1e-255,1e-254,
            1e-253,1e-252,1e-251,1e-250,1e-249,1e-248,1e-247,1e-246,1e-245,1e-244,
            1e-243,1e-242,1e-241,1e-240,1e-239,1e-238,1e-237,1e-236,1e-235,1e-234,
            1e-233,1e-232,1e-231,1e-230,1e-229,1e-228,1e-227,1e-226,1e-225,1e-224,
            1e-223,1e-222,1e-221,1e-220,1e-219,1e-218,1e-217,1e-216,1e-215,1e-214,
            1e-213,1e-212,1e-211,1e-210,1e-209,1e-208,1e-207,1e-206,1e-205,1e-204,
            1e-203,1e-202,1e-201,1e-200,1e-199,1e-198,1e-197,1e-196,1e-195,1e-194,
            1e-193,1e-192,1e-191,1e-190,1e-189,1e-188,1e-187,1e-186,1e-185,1e-184,
            1e-183,1e-182,1e-181,1e-180,1e-179,1e-178,1e-177,1e-176,1e-175,1e-174,
            1e-173,1e-172,1e-171,1e-170,1e-169,1e-168,1e-167,1e-166,1e-165,1e-164,
            1e-163,1e-162,1e-161,1e-160,1e-159,1e-158,1e-157,1e-156,1e-155,1e-154,
            1e-153,1e-152,1e-151,1e-150,1e-149,1e-148,1e-147,1e-146,1e-145,1e-144,
            1e-143,1e-142,1e-141,1e-140,1e-139,1e-138,1e-137,1e-136,1e-135,1e-134,
            1e-133,1e-132,1e-131,1e-130,1e-129,1e-128,1e-127,1e-126,1e-125,1e-124,
            1e-123,1e-122,1e-121,1e-120,1e-119,1e-118,1e-117,1e-116,1e-115,1e-114,
            1e-113,1e-112,1e-111,1e-110,1e-109,1e-108,1e-107,1e-106,1e-105,1e-104,
            1e-103,1e-102,1e-101,1e-100,1e-99,1e-98,1e-97,1e-96,1e-95,1e-94,1e-93,
            1e-92,1e-91,1e-90,1e-89,1e-88,1e-87,1e-86,1e-85,1e-84,1e-83,1e-82,1e-81,
            1e-80,1e-79,1e-78,1e-77,1e-76,1e-75,1e-74,1e-73,1e-72,1e-71,1e-70,1e-69,
            1e-68,1e-67,1e-66,1e-65,1e-64,1e-63,1e-62,1e-61,1e-60,1e-59,1e-58,1e-57,
            1e-56,1e-55,1e-54,1e-53,1e-52,1e-51,1e-50,1e-49,1e-48,1e-47,1e-46,1e-45,
            1e-44,1e-43,1e-42,1e-41,1e-40,1e-39,1e-38,1e-37,1e-36,1e-35,1e-34,1e-33,
            1e-32,1e-31,1e-30,1e-29,1e-28,1e-27,1e-26,1e-25,1e-24,1e-23,1e-22,1e-21,
            1e-20,1e-19,1e-18,1e-17,1e-16,1e-15,1e-14,1e-13,1e-12,1e-11,1e-10,1e-9,
            1e-8,1e-7,1e-6,1e-5,1e-4,1e-3,1e-2,1e-1,1e0,1e1,1e2,1e3,1e4,1e5,1e6,1e7,
            1e8,1e9,1e10,1e11,1e12,1e13,1e14,1e15,1e16,1e17,1e18,1e19,1e20,1e21,
            1e22,1e23,1e24,1e25,1e26,1e27,1e28,1e29,1e30,1e31,1e32,1e33,1e34,1e35,
            1e36,1e37,1e38,1e39,1e40,1e41,1e42,1e43,1e44,1e45,1e46,1e47,1e48,1e49,
            1e50,1e51,1e52,1e53,1e54,1e55,1e56,1e57,1e58,1e59,1e60,1e61,1e62,1e63,
            1e64,1e65,1e66,1e67,1e68,1e69,1e70,1e71,1e72,1e73,1e74,1e75,1e76,1e77,
            1e78,1e79,1e80,1e81,1e82,1e83,1e84,1e85,1e86,1e87,1e88,1e89,1e90,1e91,
            1e92,1e93,1e94,1e95,1e96,1e97,1e98,1e99,1e100,1e101,1e102,1e103,1e104,
            1e105,1e106,1e107,1e108,1e109,1e110,1e111,1e112,1e113,1e114,1e115,1e116,
            1e117,1e118,1e119,1e120,1e121,1e122,1e123,1e124,1e125,1e126,1e127,1e128,
            1e129,1e130,1e131,1e132,1e133,1e134,1e135,1e136,1e137,1e138,1e139,1e140,
            1e141,1e142,1e143,1e144,1e145,1e146,1e147,1e148,1e149,1e150,1e151,1e152,
            1e153,1e154,1e155,1e156,1e157,1e158,1e159,1e160,1e161,1e162,1e163,1e164,
            1e165,1e166,1e167,1e168,1e169,1e170,1e171,1e172,1e173,1e174,1e175,1e176,
            1e177,1e178,1e179,1e180,1e181,1e182,1e183,1e184,1e185,1e186,1e187,1e188,
            1e189,1e190,1e191,1e192,1e193,1e194,1e195,1e196,1e197,1e198,1e199,1e200,
            1e201,1e202,1e203,1e204,1e205,1e206,1e207,1e208,1e209,1e210,1e211,1e212,
            1e213,1e214,1e215,1e216,1e217,1e218,1e219,1e220,1e221,1e222,1e223,1e224,
            1e225,1e226,1e227,1e228,1e229,1e230,1e231,1e232,1e233,1e234,1e235,1e236,
            1e237,1e238,1e239,1e240,1e241,1e242,1e243,1e244,1e245,1e246,1e247,1e248,
            1e249,1e250,1e251,1e252,1e253,1e254,1e255,1e256,1e257,1e258,1e259,1e260,
            1e261,1e262,1e263,1e264,1e265,1e266,1e267,1e268,1e269,1e270,1e271,1e272,
            1e273,1e274,1e275,1e276,1e277,1e278,1e279,1e280,1e281,1e282,1e283,1e284,
            1e285,1e286,1e287,1e288,1e289,1e290,1e291,1e292,1e293,1e294,1e295,1e296,
            1e297,1e298,1e299,1e300,1e301,1e302,1e303,1e304,1e305,1e306,1e307,1e308
        };
        // clang-format on

        return constants[exponent + 323];
    }

    std::pair<char*, internal::tag> parse_number(char* p) {
        using internal::tag;

        bool negative = false;
        if ('-' == *p) {
            ++p;
            negative = true;

            if (SAJSON_UNLIKELY(at_eof(p))) {
                return std::make_pair(
                    make_error(p, ERROR_UNEXPECTED_END), tag::null);
            }
        }

        bool try_double = false;

        int i = 0;
        double d = 0.0; // gcc complains that d might be used uninitialized
                        // which isn't true. appease the warning anyway.
        if (*p == '0') {
            ++p;
            if (SAJSON_UNLIKELY(at_eof(p))) {
                return std::make_pair(
                    make_error(p, ERROR_UNEXPECTED_END), tag::null);
            }
        } else {
            unsigned char c = *p;
            if (c < '0' || c > '9') {
                return std::make_pair(
                    make_error(p, ERROR_INVALID_NUMBER), tag::null);
            }

            do {
                ++p;
                if (SAJSON_UNLIKELY(at_eof(p))) {
                    return std::make_pair(
                        make_error(p, ERROR_UNEXPECTED_END), tag::null);
                }

                unsigned char digit = c - '0';

                if (SAJSON_UNLIKELY(!try_double && i > INT_MAX / 10 - 9)) {
                    // TODO: could split this into two loops
                    try_double = true;
                    d = i;
                }
                if (SAJSON_UNLIKELY(try_double)) {
                    d = 10.0 * d + digit;
                } else {
                    i = 10 * i + digit;
                }

                c = *p;
            } while (c >= '0' && c <= '9');
        }

        int64_t exponent = 0;

        if ('.' == *p) {
            if (!try_double) {
                try_double = true;
                d = i;
            }
            ++p;
            if (SAJSON_UNLIKELY(at_eof(p))) {
                return std::make_pair(
                    make_error(p, ERROR_UNEXPECTED_END), tag::null);
            }
            char c = *p;
            if (c < '0' || c > '9') {
                return std::make_pair(
                    make_error(p, ERROR_INVALID_NUMBER), tag::null);
            }

            do {
                ++p;
                if (SAJSON_UNLIKELY(at_eof(p))) {
                    return std::make_pair(
                        make_error(p, ERROR_UNEXPECTED_END), tag::null);
                }
                d = d * 10 + (c - '0');
                // One option to avoid underflow would be to clamp
                // to INT_MIN, but int64 subtraction is cheap and
                // in the absurd case of parsing 2 GB of digits
                // with an extremely high exponent, this will
                // produce accurate results.  Instead, we just
                // leave exponent as int64_t and it will never
                // underflow.
                --exponent;

                c = *p;
            } while (c >= '0' && c <= '9');
        }

        char e = *p;
        if ('e' == e || 'E' == e) {
            if (!try_double) {
                try_double = true;
                d = i;
            }
            ++p;
            if (SAJSON_UNLIKELY(at_eof(p))) {
                return std::make_pair(
                    make_error(p, ERROR_UNEXPECTED_END), tag::null);
            }

            bool negativeExponent = false;
            if ('-' == *p) {
                negativeExponent = true;
                ++p;
                if (SAJSON_UNLIKELY(at_eof(p))) {
                    return std::make_pair(
                        make_error(p, ERROR_UNEXPECTED_END), tag::null);
                }
            } else if ('+' == *p) {
                ++p;
                if (SAJSON_UNLIKELY(at_eof(p))) {
                    return std::make_pair(
                        make_error(p, ERROR_UNEXPECTED_END), tag::null);
                }
            }

            int exp = 0;

            char c = *p;
            if (SAJSON_UNLIKELY(c < '0' || c > '9')) {
                return std::make_pair(
                    make_error(p, ERROR_MISSING_EXPONENT), tag::null);
            }
            for (;;) {
                // c guaranteed to be between '0' and '9', inclusive
                unsigned char digit = c - '0';
                if (exp > (INT_MAX - digit) / 10) {
                    // The exponent overflowed.  Keep parsing, but
                    // it will definitely be out of range when
                    // pow10 is called.
                    exp = INT_MAX;
                } else {
                    exp = 10 * exp + digit;
                }

                ++p;
                if (SAJSON_UNLIKELY(at_eof(p))) {
                    return std::make_pair(
                        make_error(p, ERROR_UNEXPECTED_END), tag::null);
                }

                c = *p;
                if (c < '0' || c > '9') {
                    break;
                }
            }
            static_assert(
                -INT_MAX >= INT_MIN, "exp can be negated without loss or UB");
            exponent += (negativeExponent ? -exp : exp);
        }

        if (exponent) {
            assert(try_double);
            // If d is zero but the exponent is huge, don't
            // multiply zero by inf which gives nan.
            if (d != 0.0) {
                d *= pow10(exponent);
            }
        }

        if (negative) {
            if (try_double) {
                d = -d;
            } else {
                i = -i;
            }
        }
        if (try_double) {
            bool success;
            size_t* out
                = allocator.reserve(double_storage::word_length, &success);
            if (SAJSON_UNLIKELY(!success)) {
                return std::make_pair(oom(p, "double"), tag::null);
            }
            double_storage::store(out, d);
            return std::make_pair(p, tag::double_);
        } else {
            bool success;
            size_t* out
                = allocator.reserve(integer_storage::word_length, &success);
            if (SAJSON_UNLIKELY(!success)) {
                return std::make_pair(oom(p, "integer"), tag::null);
            }
            integer_storage::store(out, i);
            return std::make_pair(p, tag::integer);
        }
    }

    bool install_array(size_t* array_base, size_t* array_end) {
        using namespace sajson::internal;

        const size_t length = array_end - array_base;
        bool success;
        size_t* const new_base = allocator.reserve(length + 1, &success);
        if (SAJSON_UNLIKELY(!success)) {
            return false;
        }
        size_t* out = new_base + length + 1;
        size_t* const structure_end = allocator.get_write_pointer_of(0);

        while (array_end > array_base) {
            size_t element = *--array_end;
            tag element_type = get_element_tag(element);
            size_t element_value = get_element_value(element);
            size_t* element_ptr = structure_end - element_value;
            *--out = make_element(element_type, element_ptr - new_base);
        }
        *--out = length;
        return true;
    }

    bool install_object(size_t* object_base, size_t* object_end) {
        using namespace internal;

        assert((object_end - object_base) % 3 == 0);
        const size_t length_times_3 = object_end - object_base;
        const size_t length = length_times_3 / 3;
        if (SAJSON_UNLIKELY(should_binary_search(length))) {
            std::sort(
                reinterpret_cast<object_key_record*>(object_base),
                reinterpret_cast<object_key_record*>(object_end),
                object_key_comparator(input.get_data()));
        }

        bool success;
        size_t* const new_base
            = allocator.reserve(length_times_3 + 1, &success);
        if (SAJSON_UNLIKELY(!success)) {
            return false;
        }
        size_t* out = new_base + length_times_3 + 1;
        size_t* const structure_end = allocator.get_write_pointer_of(0);

        while (object_end > object_base) {
            size_t element = *--object_end;
            tag element_type = get_element_tag(element);
            size_t element_value = get_element_value(element);
            size_t* element_ptr = structure_end - element_value;

            *--out = make_element(element_type, element_ptr - new_base);
            *--out = *--object_end;
            *--out = *--object_end;
        }
        *--out = length;
        return true;
    }

    char* parse_string(char* p, size_t* tag) {
        using namespace internal;

        ++p; // "
        size_t start = p - input.get_data();
        char* input_end_local = input_end;
        while (input_end_local - p >= 4) {
            if (!is_plain_string_character(p[0])) {
                goto found;
            }
            if (!is_plain_string_character(p[1])) {
                p += 1;
                goto found;
            }
            if (!is_plain_string_character(p[2])) {
                p += 2;
                goto found;
            }
            if (!is_plain_string_character(p[3])) {
                p += 3;
                goto found;
            }
            p += 4;
        }
        for (;;) {
            if (SAJSON_UNLIKELY(p >= input_end_local)) {
                return make_error(p, ERROR_UNEXPECTED_END);
            }

            if (!is_plain_string_character(*p)) {
                break;
            }

            ++p;
        }
    found:
        if (SAJSON_LIKELY(*p == '"')) {
            tag[0] = start;
            tag[1] = p - input.get_data();
            *p = '\0';
            return p + 1;
        }

        if (*p >= 0 && *p < 0x20) {
            return make_error(p, ERROR_ILLEGAL_CODEPOINT, static_cast<int>(*p));
        } else {
            // backslash or >0x7f
            return parse_string_slow(p, tag, start);
        }
    }

    char* read_hex(char* p, unsigned& u) {
        unsigned v = 0;
        int i = 4;
        while (i--) {
            unsigned char c = *p++;
            if (c >= '0' && c <= '9') {
                c -= '0';
            } else if (c >= 'a' && c <= 'f') {
                c = c - 'a' + 10;
            } else if (c >= 'A' && c <= 'F') {
                c = c - 'A' + 10;
            } else {
                return make_error(p, ERROR_INVALID_UNICODE_ESCAPE);
            }
            v = (v << 4) + c;
        }

        u = v;
        return p;
    }

    void write_utf8(unsigned codepoint, char*& end) {
        if (codepoint < 0x80) {
            *end++ = codepoint;
        } else if (codepoint < 0x800) {
            *end++ = 0xC0 | (codepoint >> 6);
            *end++ = 0x80 | (codepoint & 0x3F);
        } else if (codepoint < 0x10000) {
            *end++ = 0xE0 | (codepoint >> 12);
            *end++ = 0x80 | ((codepoint >> 6) & 0x3F);
            *end++ = 0x80 | (codepoint & 0x3F);
        } else {
            assert(codepoint < 0x200000);
            *end++ = 0xF0 | (codepoint >> 18);
            *end++ = 0x80 | ((codepoint >> 12) & 0x3F);
            *end++ = 0x80 | ((codepoint >> 6) & 0x3F);
            *end++ = 0x80 | (codepoint & 0x3F);
        }
    }

    char* parse_string_slow(char* p, size_t* tag, size_t start) {
        char* end = p;
        char* input_end_local = input_end;

        for (;;) {
            if (SAJSON_UNLIKELY(p >= input_end_local)) {
                return make_error(p, ERROR_UNEXPECTED_END);
            }

            if (SAJSON_UNLIKELY(*p >= 0 && *p < 0x20)) {
                return make_error(
                    p, ERROR_ILLEGAL_CODEPOINT, static_cast<int>(*p));
            }

            switch (*p) {
            case '"':
                tag[0] = start;
                tag[1] = end - input.get_data();
                *end = '\0';
                return p + 1;

            case '\\':
                ++p;
                if (SAJSON_UNLIKELY(p >= input_end_local)) {
                    return make_error(p, ERROR_UNEXPECTED_END);
                }

                char replacement;
                switch (*p) {
                case '"':
                    replacement = '"';
                    goto replace;
                case '\\':
                    replacement = '\\';
                    goto replace;
                case '/':
                    replacement = '/';
                    goto replace;
                case 'b':
                    replacement = '\b';
                    goto replace;
                case 'f':
                    replacement = '\f';
                    goto replace;
                case 'n':
                    replacement = '\n';
                    goto replace;
                case 'r':
                    replacement = '\r';
                    goto replace;
                case 't':
                    replacement = '\t';
                    goto replace;
                replace:
                    *end++ = replacement;
                    ++p;
                    break;
                case 'u': {
                    ++p;
                    if (SAJSON_UNLIKELY(!has_remaining_characters(p, 4))) {
                        return make_error(p, ERROR_UNEXPECTED_END);
                    }
                    unsigned u = 0; // gcc's complaining that this could be used
                                    // uninitialized. wrong.
                    p = read_hex(p, u);
                    if (!p) {
                        return 0;
                    }
                    if (u >= 0xD800 && u <= 0xDBFF) {
                        if (SAJSON_UNLIKELY(!has_remaining_characters(p, 6))) {
                            return make_error(p, ERROR_UNEXPECTED_END_OF_UTF16);
                        }
                        char p0 = p[0];
                        char p1 = p[1];
                        if (p0 != '\\' || p1 != 'u') {
                            return make_error(p, ERROR_EXPECTED_U);
                        }
                        p += 2;
                        unsigned v = 0; // gcc's complaining that this could be
                                        // used uninitialized. wrong.
                        p = read_hex(p, v);
                        if (!p) {
                            return p;
                        }

                        if (v < 0xDC00 || v > 0xDFFF) {
                            return make_error(
                                p, ERROR_INVALID_UTF16_TRAIL_SURROGATE);
                        }
                        u = 0x10000 + (((u - 0xD800) << 10) | (v - 0xDC00));
                    }
                    write_utf8(u, end);
                    break;
                }
                default:
                    return make_error(p, ERROR_UNKNOWN_ESCAPE);
                }
                break;

            default:
                // validate UTF-8
                unsigned char c0 = p[0];
                if (c0 < 128) {
                    *end++ = *p++;
                } else if (c0 < 224) {
                    if (SAJSON_UNLIKELY(!has_remaining_characters(p, 2))) {
                        return unexpected_end(p);
                    }
                    unsigned char c1 = p[1];
                    if (c1 < 128 || c1 >= 192) {
                        return make_error(p + 1, ERROR_INVALID_UTF8);
                    }
                    end[0] = c0;
                    end[1] = c1;
                    end += 2;
                    p += 2;
                } else if (c0 < 240) {
                    if (SAJSON_UNLIKELY(!has_remaining_characters(p, 3))) {
                        return unexpected_end(p);
                    }
                    unsigned char c1 = p[1];
                    if (c1 < 128 || c1 >= 192) {
                        return make_error(p + 1, ERROR_INVALID_UTF8);
                    }
                    unsigned char c2 = p[2];
                    if (c2 < 128 || c2 >= 192) {
                        return make_error(p + 2, ERROR_INVALID_UTF8);
                    }
                    end[0] = c0;
                    end[1] = c1;
                    end[2] = c2;
                    end += 3;
                    p += 3;
                } else if (c0 < 248) {
                    if (SAJSON_UNLIKELY(!has_remaining_characters(p, 4))) {
                        return unexpected_end(p);
                    }
                    unsigned char c1 = p[1];
                    if (c1 < 128 || c1 >= 192) {
                        return make_error(p + 1, ERROR_INVALID_UTF8);
                    }
                    unsigned char c2 = p[2];
                    if (c2 < 128 || c2 >= 192) {
                        return make_error(p + 2, ERROR_INVALID_UTF8);
                    }
                    unsigned char c3 = p[3];
                    if (c3 < 128 || c3 >= 192) {
                        return make_error(p + 3, ERROR_INVALID_UTF8);
                    }
                    end[0] = c0;
                    end[1] = c1;
                    end[2] = c2;
                    end[3] = c3;
                    end += 4;
                    p += 4;
                } else {
                    return make_error(p, ERROR_INVALID_UTF8);
                }
                break;
            }
        }
    }

    mutable_string_view input;
    char* const input_end;
    Allocator allocator;

    internal::tag root_tag;
    size_t error_line;
    size_t error_column;
    error error_code;
    int error_arg; // optional argument for the error
};
/// \endcond

/**
 * Parses a string of JSON bytes into a \ref document, given an allocation
 * strategy instance.  Any kind of string type is valid as long as a
 * mutable_string_view can be constructed from it.
 *
 * Valid allocation strategies are \ref single_allocation,
 * \ref dynamic_allocation, and \ref bounded_allocation.
 *
 * A \ref document is returned whether or not the parse succeeds: success
 * state is available by calling document::is_valid().
 */
template <typename AllocationStrategy, typename StringType>
document parse(const AllocationStrategy& strategy, const StringType& string) {
    mutable_string_view input(string);

    bool success;
    auto allocator = strategy.make_allocator(input.length(), &success);
    if (!success) {
        return document(input, 1, 1, ERROR_OUT_OF_MEMORY, 0);
    }

    return parser<typename AllocationStrategy::allocator>(
               input, std::move(allocator))
        .get_document();
}
} // namespace sajson
